Superconducting magnets, such as those used to generate homogenous background magnetic fields in MRI systems, are usually constructed from several separate coils electrically connected in series. The magnet structure is housed within a cryostat which keeps the magnet cooled to a temperature at which the coils are superconducting. In many magnets, the coils are housed within a cryogen vessel partially filled with liquid cryogen, and partially filled with a gaseous cryogen.
It is desirable to have electrical access to each coil junction in order to measure quench voltages and diagnose which coil or section of coil quenched first. Known systems have provided simple electrical connections from nodes between coils to simple high-voltage lead-throughs, arranged to carry voltages of up to several kilovolts to the exterior of the cryostat. Since these voltages can reach several kilovolts, appropriate insulation in a gaseous cryogen atmosphere, for example helium, is extremely challenging and dangerous. Helium gas has a much lower breakdown voltage than air, so the required standard of insulation is much greater than would be the case in air. Another disadvantage of such an arrangement is that the necessary high voltage connectors introduce a significant heat leak path into the cryostat.
This problem has conventionally been addressed by providing a voltage divider near each node between coils. FIG. 5 illustrates such a conventional arrangement. A node 14 between coils 10, 12, is electrically connected to a first resistor R1. R1 is connected to a ground voltage, typically the voltage of the cryostat body, through a second resistor R2. Resistor R1 is of much larger value than R2, for example R1=100MΩ, R2=100 kΩ. Node 22, between R1 and R2, is electrically connected to a connector 16, which carries the voltage at node 22 through the boundary 18 of the cryostat. A voltage measuring device 24, such as a data logger, may be attached to the connector 16 on the outside of the cryostat, to measure voltages at the connector.
The resistors R1 and R2, forming the voltage divider, are typically placed on a small circuit board close to the node 14 between coils. Any voltage Vin derived from the node 14 is divided in proportion to resistors R1 and R2, and a fraction of that voltage Vin is supplied to the connector 16. In the illustrated embodiment, R1=100MΩ, R2=100 kΩ and so the voltage Vout appearing at the voltage measuring device is about one one-thousandth of the voltage at the node 14, that is, Vout≈Vin/1000. Such large value resistors are used to limit the current which may flow through the connector. If a 5 kV voltage is applied through a 100MΩ resistor to ground, a current of only 50 μA will flow, which is considered safe.
Such an arrangement ensures that even voltages of 5 kV, which may arise during a quench, are scaled to a safe value of 5V before they leave the cryogen vessel. While such high voltages are scaled to a small, safe value, any small voltages Vin which may be generated, by coil movement for instance, will also be reduced by the same ratio when measured.
It is known that analyzing the small voltages generated by many phenomena and fault modes such as: accurate individual coil ramp voltages, coil movement, field decay, short circuits, flux jumps and thermal quench initiation, can reveal a great deal of information leading to accurate diagnosis.
This information is not available after attenuation in the manner shown in FIG. 5 as real signals Vout≈Vin/1000 derived from such small voltages are lost in electrical and thermal noise of similar magnitude and also present in the attenuated voltages Vout presented for measurement.